Aerogel sorbents

ABSTRACT

The current invention describes methods and compositions of various sorbents based on aerogels of various silanes and their use as sorbent for carbon dioxide. Methods further provide for optimizing the compositions to increase the stability of the sorbents for prolonged use as carbon dioxide capture matrices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit from U.S. Provisional PatentApplication 61/639,893 filed on Apr. 28, 2012, which is herebyincorporated by reference in its entirety as if sully set forth.

GOVERNMENT INTEREST

This invention was made with the United States Government support underthe Contract no. DE-SC0004289 from the U.S. Department of Energy. TheGovernment has certain rights in the invention.

DESCRIPTION OF THE INVENTION

Climate change and global warming is currently considered one of themost pressing and severe environmental problems. One of the main causesfor global warming is believed to be the increasing concentration ofcarbon dioxide in the atmosphere due to the combustion of fossil fuelssuch as coal, petroleum and natural gas. Fossil fuels supply about 85%of the world's energy needs since fossil fuels are still relativelyinexpensive and easy to use, and no satisfactory alternatives areavailable to replace them on the enormous scale needed. The increasinguse of fossil fuels releases huge amounts of carbon dioxide in theatmosphere every year, and there is increased pressure to limit thesereleases due to its perceived impact on global climate change.

Most of the emissions of CO₂ to the atmosphere come from the electricitygeneration power plants and the industrial sector and are bi-productsfrom combustion of fossil fuels. The CO₂ concentration in flue gas istypically 4-14% by volume, although CO₂ is produced at higherconcentrations by a few industrial processes. In principle, flue gascould be stored, to avoid emissions of CO₂ to the atmosphere, but itwould have to be compressed to a pressure of typically more than 10 MPawhich would consume an excessive amount of energy. Also, the high volumeof flue gas would mean that storage reservoirs would be filled quickly.

The primary approach for limiting the release of CO₂ into the atmosphereis to capture it as it's released for possible storage via one ofseveral potential sequestration technologies. A relatively high purityCO₂ source is preferred for transport and sequestration, but a keyroadblock to CO₂ sequestration is the development of cost-effective CO₂capture/separation technologies. The most likely options for CO₂separation and capture include chemical absorption, physical andchemical adsorption, low-temperature distillation, gas-separationmembranes, mineralization/biomineralization, and vegetation. Viable CO₂capture and sequestration technologies would permit the world tocontinue using fossil fuels but with much reduced emissions of CO₂, andallow time for low-CO₂ emission energy sources to be developed andintroduced on a large scale.

The most widely used technology for the capture of CO₂ is the“wet-scrubbing” amine-solution based sorption process. Carbon dioxidescrubbing is currently used on a large scale for the purification ofindustrial gases (natural gas, syngas, etc.). These processes use mainlyalkanolamine aqueous solutions (G. Astarita, D. W. Savage and A. Bisio,Gas Treating with Chemical Solvents, John Wiley, NY, 1983), the mostcommon being mono- and di-ethanolamines, (MEA and DEA) andN-methyldiethanolamine (MDEA). The process is reversible and can berepresented as follows:

Wet chemical stripping of CO₂ involves one or more reversible chemicalreactions between CO₂ and another substance such as MEA to produce acarbonate. Upon heating, the carbonate decomposes to free CO₂ and theoriginal amine is regenerated which becomes available to react withadditional CO₂. An example of the process using monoethanol amine is:

HOC₂H₄NH₂+CO₂+H₂O⇄HOC₂H₄NH₃ ⁺HCO₃ ⁻

Typically, MEA and DEA are used as 25 to 30 wt. % amine in aqueoussolution. The amine solution enters the top of an absorption tower whilethe carbon dioxide containing gaseous stream is introduced at thebottom. During contact with the CO₂-containing gaseous stream, the aminesolution chemically absorbs the carbon dioxide from the gaseous stream.Desorption of the adsorbed carbon dioxide proceeds through a thermalregeneration process. Carbon dioxide and water emerge from the aminesolution and the water is separated by condensing the water vapor in aheat exchanger. After regeneration the amine solution is recycled backto the absorption tower for additional carbon dioxide absorption.However, this process has several disadvantages, such as high energyconsumption, solvent regeneration, the corrosion of the equipment andtoxicity. Further, the amine solution has a limited lifetime due todegradation through oxidation of the amine.

A promising alternative technology to the liquid-phase sorption is touse solid sorbents for capturing CO₂ by means of the pressure ortemperature swing adsorption system, offering possible energy savingsand stable performance. There are different classifications of sorbents;chemical sorbents that react with the CO₂ and physical sorbents thatadsorb the CO₂. Amines and other chemicals, such as sodium carbonate,can be immobilized on the surface of solid supports to create a sorbentthat reacts with the CO₂. Solid sorbents that physically adsorb CO₂include carbon based materials, carbon nanotubes and zeolites (naturaland synthetic). Potential advantages of solid sorbents are as follows:Ease of material handling (coal plants are experienced with solidshandling), Safe for local environment, High CO₂ capacity, Lowerregeneration energy and Multi-pollutant control.

Chemical sorbents that react with the CO₂ in the flue gas include asupport, usually high surface area, with an immobilized amine or otherreactant on the surface. The surface area allows for numerous reactionsites. Examples of commonly used supports are alumina or silica, whilecommon reactants include amines such as polyethylenimine′ or chemicalssuch as sodium carbonate.

SUMMARY OF THE INVENTION

The current invention discloses porous carbon dioxide sorbentscomprising an aerogel of products of co-gelation of at least ahydrolyzed alkylalkoxysilane and a hydrolyzed aminosilane. The aerogelsof the sorbent comprises at least an open pore accessible to carbondioxide. Preferably, the open pore is interconnected with one or more ofother pores. The aerogels involved in the current invention have asilica network created by the sol-gel reaction of a hydrolyzedalkylalkoxysilane and a hydrolyzed aminosilane. Tertraalkoxysilanes or ahydrolyzed form thereof may also be included in the sol-gel reaction toproduce complex structures with unique properties relevant to theresulting materials used as sorbents. When tetroalkoxysilanes are used,he percentage of tetraalkoxysilane sin the total silanes may vary from5% to about 90%.

The amino groups present in the sorbents are such that carbon dioxidecan reasonably access the groups and participate in the sorption. Theaerogels of the current invention may be inherently hydrophobic due tothe alkyl groups in the silica matrix or optionally silylated after thesilica matrix is formed by treating the gel matrix with a silylatingagent such as hexamethyldisilazane, hexamethuldisiloxane or others asdescribed in this document or known in the art. The sol-gel reaction orco-gelation may also include dialkyldialkoxysilane.

The alkylalkoxysilane may contain one, two or three alkyl groups. Thealkyl groups in the several silanes described herein contains between 1and 6 carbon atoms. The alkyl groups may be methyl, ethyl, propyl,n-butyl, t-butyl or other higher carbon alkyl groups. The varioussilanes used in the different embodiments of the current invention mayalso have additional functional groups. The alkylalkoxysilanes describedhere may be selected from the group consisting ofmethyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane,ethyltriethoxysilane, propyltrimethoxysilane, propyltriethoxysilane,dimethyldimethoxysilane, dimethyldiethoxysilane, diethyldiethoxysilane,diethyldimethoxysilane, trimethylmethoxysilane, trimethylethoxysilane,triethylmethoxysilane, triethylethoxysilane, tripropylmethoxysilane,tripropylethoxysilane, (3,3,3-Trifluoropropyl)trimethoxysilane,(3,3,3-Trifluoropropyl)triethoxysilane and a combination thereof. Thedialkyldialkoxysilanes described herein may be selected from the groupconsisting of dimethyldimethoxysilane, dimethyldiethoxysilane,diethyldiethoxysilane, or diethyldimethoxysilane or mixtures thereof.The aminosilanes may be a mono, di, tri or poly amine-containingsilanes. The aminosilanes may also be aminoalkoxysilanes selected fromthe group consisting of 3-aminopropyl-triethoxysilane (APTES),3-aminopropyl-trimethoxysilane (APTMS),N-(2-aminoethyl)-3-aminopropyltriethoxysilane (AE-APTES),N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AE-APTMS),p-aminophenyltrimethoxysilane,N-3-([amino(polypropylenoxy])-amino-propyl-trimethoxy-silane(aminoether), (3-trimethoxylsilylpropyl)diethylenetriamine (TMS-DETA),trimethoxy-silane modified polyethyleneimine and a combination thereof.

In an embodiment, the amino groups are located for the most part on orclose proximity to the aerogel surfaces or pore surfaces. Sorbents ofthe current invention may be contacted with a gaseous stream comprisingat least some carbon dioxide and on such exposure capture at least somecarbon dioxide from such stream in the sorbent. The sorbents of thecurrent invention may be regenerated by heating, purging with anothergas, by exposing to a reduced pressure or other means known in the artfor regenerating sorbent systems. The sorbents may be designed to bestable for at least 250 capture-regeneration cycles, preferably, for atleast 500 capture-regeneration cycles, more preferably, for at least1000 capture-regeneration cycles, or further more preferably, for atleast 2000 capture-regeneration cycles. The stability refers to thesorbent's ability to retain its original chemical compositionsubstantially at least by 60%. The sorbents also retain the capture rateof carbon dioxide for extended capture-regeneration cycles. The sorbentsmay also be exposed to high temperatures. In some cases, the degradationin the capture rate of carbon dioxide of up to 80% of the original valueor up to 90% of the original value may be tolerated. The moisturecontent of the sorbents of various embodiments may be controlled to beless than 10% by weight and preferably less than 2% by weight. Theco-gelation reactants may comprise aminosilanes from 5% to 70% byweight.

The aerogel sorbents of the current invention have densities between0.01 and 0.6 g/cc and preferably between 0.03 and 0.34 g/cc. Thesorbents of various embodiments of the current invention have the carbondioxide capture rate between 0.08 g and 0.4 g of carbon dioxide per gramof the aerogel in the sorbent. In some embodiments, the capture rate ofthe sorbents degrades no more than 80% while exposed to temperatures upto 130° C. and capture-regeneration cycles of at least 500.

The process of preparing the carbon dioxide capture sorbent involves thesteps of co-gelling at least a hydrolyzed alkylalkoxysilane with ahydrolyzed aminosilane. Various compatible solvents are useful in theco-gelation step including alcohols containing between 1 and 6 carbonatoms. In a preferred embodiment, ethanol is used as the solvent. Wateris various ratios may also be involved to accomplish the sol-gelreactions. Water may also be a product of some of these reactions. Theratios and solvents are adjusted so that precipitation or phaseseparation is avoided. Tertraalkoxysilanes or a hydrolyzed form thereofmay also be included in the sol-gel reaction to produce complexstructures with unique properties relevant to the resulting materialsused as sorbents. When tetroalkoxysilanes are used, he percentage oftetraalkoxysilane sin the total silanes may vary from 5% to about 90%.

The resultant gel is dried to obtain an aerogel. The drying may beperformed in various ways, including supercritical drying, usingsupercritical carbon dioxide, ambient or slightly elevated pressuredrying, subcritical drying, freeze drying or various combinationsthereof. The wet gels or the dried aerogels may be treated withsilylating agents such as hexamethyldisilazane or hexamethyldisiloxane.In a preferred embodiment, a wet gel is treated with the silylatingagent and dried through the various methods described above. Optionally,tetra-alkoxysilane is included in the co-gelation. In yet anotherembodiment, dialkyldialkoxysilane in various amounts is added to theco-gelation. The alkyl groups of various silanes used in these processescontain between 1 and 6 carbon atoms. The silanes may also haveadditional functional groups. The alkylalkoxysilanes used in the variousprocesses may contain mono, di or tri alkly groups and may be selectedfrom the group consisting of methyltrimethoxysilane,methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane,propyltrimethoxysilane, propyltriethoxysilane, dimethyldimethoxysilane,dimethyldiethoxysilane, diethyldiethoxysilane, diethyldimethoxysilane,trimethylmethoxysilane, trimethylethoxysilane, triethylmethoxysilane,triethylethoxysilane, tripropylmethoxysilane, tripropylethoxysilane,(3,3,3-Trifluoropropyl)trimethoxysilane,(3,3,3-Trifluoropropyl)triethoxysilane and a combination thereof. Theaminosilans may be mono, di, tri or poly amine-containing silanes. Theaminosilanes may also contain alkoxy groups such that they areamonoalkoxysilanes.

The aminosilane may be selected from the group consisting of3-aminopropylmethyldiethoxysilane, 3-aminopropyl-triethoxysilane(APTES), 3-aminopropyl-trimethoxysilane (APTMS),N-(2-aminoethyl)-3-aminopropyltriethoxysilane (AE-APTES),N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AE-APTMS),p-aminophenyltrimethoxysilane,N-3-([amino(poly-propyleneoxy])-amino-propyl-trimethoxy-silane(aminoether), (3-trimethoxylsilylpropyl)-diethylenetriamine (TMS-DETA),trimethoxy-silane modified polyethyleneimine and a combination thereof.The aminogroups are preferably located on the surfaces of the resultingaerogels.

The various sorbents described herein may be used to capture carbondioxide either in a gaseous stream or present in a defined space. Thesorbents may be ground into powders with particles sizes in the range of1 microns to 10 mm. The ground aerogels may be used in sorbing carbondioxide. Such ground aerogels may be present in columns where a gaseousstream containing carbon dioxide is flown through. After the sorbentshave sufficiently captured the carbon dioxide, they may be heated,purged with another gas or subjected to a reduced pressure to remove thecarbon dioxide. The capture and removal of carbon dioxide may beperformed at different locations resulting in the segregation orsequestration of carbon dioxide. The removal or carbon dioxide from thesorbents by heating, purging with another gas or subjecting o a reducedpressure is also referred to as regeneration.

The process of capturing the carbon dioxide and regenerating the sorbentsuch that it is ready for use as sorbent again is referred to ascapture-regeneration cycle. The sorbents produced in the variousprocesses described here may be stable for at least 250capture-regeneration cycles, preferably for at least 500capture-regeneration cycles and more preferably, for at least 1000capture-regeneration cycles and still more preferably for at least 2000capture-regeneration cycles. The co-gelation reactants of variousprocesses described here may have aminosilanes from 5% to 70% by weight.

A process of capturing carbon dioxide comprising the following steps isdescribed. A carbon dioxide sorbent described in the various embodimentsof the current invention are provided in monolithic, composite or powderform. At least part of a gas or gaseous stream is contacted with thesorbent. The resulting sorbent is heated, purged with another gas orsubjected to reduced pressure to remove the carbon dioxide. The removedcarbon dioxide is collected and transported or stored as appropriate. Inanother embodiment, a process is described where a gel is formed fromthe gelation of alkyltrialkoxysilane or co-gelation ofalkyltrialkoxysilane and tetraalkoxysilane and the resulting gel isexposed to an amine and subsequently dried. The sorbent obtained in sucha process is used to capture carbon dioxide as described above. Theamines used for amine treatment of the gel as described above isselected from tetraethylenepentamine (TEPA), polyethyleneimine (PEI) orcombinations thereof.

An object of the current invention is to produce amine functionalizedhydrophobic silica aerogels for CO₂ sorption and use thereof. Inaccordance with one embodiment, the present invention provides amoisture stable CO₂ adsorbent where loss of the amine and CO₂ adsorptioncapacity can largely or completely be prevented. One embodiment ensuresmoisture stability by bonding the amine into the backbone of anintrinsically hydrophobic methyltriethoxysilane (MTES) aerogel and it'sCO₂ adsorption capacity can exceed 10 wt % CO₂ at 40° C.

In accordance with one aspect of the present invention there is providedan aerogel support having an open interconnected pore structure thatallows facile access of gases to the pore surfaces. The silica aerogelhas a relatively broad pore size distribution compared to periodicsilica supports previously disclosed and can have pores as large as 100nm. The aerogels of the current invention have a pore volume of between0.2 and 1.6 cc/g, average pore diameter of between 3 and 40 nm, and aBET surface area of between 20 and 500 m²/g.

In accordance with another embodiment of the present invention, there isprovided an inherently hydrophobic, regenerable sorbent for use in a CO₂capture process, where said sorbent is comprised of anamine-functionalized silica aerogel, wherein amino groups are covalentlybonded to the surface or network structure and are readily accessiblewithin the interconnected pores or pore walls of the silica aerogel.

In accordance with another embodiment of the present invention there isprovided a method for removing or recovering carbon dioxide from angaseous stream or atmosphere containing carbon dioxide, comprising thestep of contacting the gaseous stream or atmosphere with an adsorbentcomprising a functionalized aerogel support having a pore volume ofbetween 0.2 and 1.6 cc/g, a median pore diameter of between 4 and 40 nm,and a BET surface area of between 20 and 500 m²/g, which support isfunctionalized by addition of CO₂ reactive functional groups within thepores and external surface of said support material.

In accordance with another embodiment of the present invention there isprovided a process for manufacturing an adsorbent, comprising: (a)providing a functionalized aerogel support having a pore volume ofbetween 0.2 and 1.6 cc/g, a median pore diameter of between 4 and 40 nm,and a BET surface area of between 20 and 500 m²/g; and (b) grafting afunctionalization compound, which contains one or more CO₂ reactivegroups, to the surface of the pores of said support material; or (c)directly loading a functionalization compound, which contains one ormore CO₂ reactive groups, into the pores of said support material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates structure of some of the amine alkoxysilaneprecursors used in sol gel synthesis of this program.

FIG. 2 illustrates a synthesis route to prepare Amine FunctionalizedAerogels for CO₂ capture.

FIG. 3 illustrates nitrogen sorption-desorption isotherms ofmono-amine/MTES aerogels (no ammonia added).

FIG. 4 illustrates nitrogen sorption-desorption isotherms ofmono-amine/MTES aerogels (ammonia added).

FIG. 5 illustrates pore size distribution of mono-amine/MTES aerogels(no ammonia added).

FIG. 6 illustrates pore size distribution of mono-amine/MTES aerogels(no ammonia added).

FIG. 7 illustrates an XPS spectra of MTES aerogel (blank sample) and twoamine functionalized aerogels.

FIG. 8 illustrates a schematic of CO2 sorbent test unit.

FIG. 9 illustrates the performance of some amine functionalized aerogelsof the current invention

FIG. 10 illustrates the working capacity of one of the samples.

FIG. 11 illustrates the CO₂ adsorption capacity at P_(CO2)=0.15 versustemperature.

FIG. 12 illustrates the working capacity of another of the samples.

FIG. 13 illustrates CO₂ sorption/desorption capacity over 275 cycles ofa sample.

FIG. 14 illustrates a comparison of cyclic stability of an AFA sorbentand a typical amine supported sorbent.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to efficient removal of carbon dioxide,for example, from dry or humid process streams, or atmospheres, byselective sorption using a sorbent comprising an amine functionalizedsilica aerogel support material. In a specific embodiment, the supportmaterial is characterized by a total pore volume of between 0.2 to 1.6cc/g, a median pore diameter of 4 to 40 nm, and a total BET surface areaof between 20 and 500 m²/g. The pore diameter of the support material ofthe present invention is larger than in previously known typicalperiodic mesoporous silica support materials (e.g. KIT-n, MCM-41, SBA-nand MCM-48) and is characterized by having an interconnected openporosity instead of non-interconnected channels.

The sorbent of the present invention is prepared using various methods,including those outlined herein, in order to obtain material havingvarying capacities and rates of sorption. In each case the sorbentcomprises a hydrophobic silica aerogel that contains amine functionalgroups that remain accessible to the sorbate.

The current patent application makes use of an inherently hydrophobicaerogel which also contains amines bonded to the surface of the porewalls and/or within the silica matrix. Compared to periodic silicasupports and fumed silica they are unique materials and arecharacterized by their high surface areas and interconnected open poreswhich allows facile access of gases to all pores throughout the porousstructure.

Aerogels are highly porous low-density materials, prepared by forming agel and subsequently eliminating the liquid with preservation of the gelstructure. In a narrow sense, an aerogel is understood to refer to amaterial in which the liquid has been removed from the gel undersupercritical conditions or using a supercritical fluid, whereas, whenthe gel is dried under subcritical conditions, the resulting product iscalled a xerogel, and when the liquid is eliminated from the frozenstate by sublimation, the product is called a cryogel. Aerogels in abroad sense are understood to have any porous gel structure wheresolvents are replaced with air or another gas or combination of gases.In the broad sense aerogels include the aerogels, xerogels and cryogelsin the narrow sense. The word “supercritically dried” as used hereinrefers to the processes where the gels are dried at or above thesupercritical condition of the solvent involved or dried using asupercritical fluid like supercritical carbon dioxide. While theconditions might be supercritical for the solvent, for example, CO₂, itneed not be above the critical point of the mixture, say a mixture ofalcohol and CO₂.

Because of their high porosity, aerogels have interesting physicalproperties which make them suitable for use, among other things, as heatinsulating materials, acoustic materials, luminescent solar collectors,gas filters, catalysts or support materials.

In general, the aerogels used are those based on metal oxides suitablefor the sol-gel technology (see e.g., C. J. Brinker and G. W. Scherer,Sol-Gel Science, 1990, Chapters 2 and 3), such as Si, Al, Ti, Sn or Zrcompounds, or those based on organic substances suitable for the sol-geltechnology, such as melamine-formaldehyde condensates (U.S. Pat. No.5,086,085) or resorcinol-formaldehyde condensates (U.S. Pat. No.4,873,218). However, they can also be based on mixtures of theaforementioned materials. Used by preference are aerogels containing Sior Al compounds, particularly Si compounds; SiO₂ aerogels areparticularly preferred. Aerogels can be prepared from mixtures of puremetal alcoholates, particularly of Si, Al, Zr, Ti and Sn alcoholates ormixtures thereof. Here, the term “metal alcoholate” includes thecorresponding semimetal and in this invention the gel is preferablyprepared from mixtures of tetraalkoxysilanes [Si(OR)₄, wherein Rrepresents C₁-C₆-alkyl moiety, preferably methyl or ethyl, andtrialkoxysilanes [(RO)₃Si—R′, where R′ represents C₁-C₆-alkyl moiety(preferably methyl), and R has the meaning indicated above]. Suchalkoxysilanes (methoxy, ethoxy, propoxy, butoxysilane, etc) arecommercially available. However there continues to be a demand foradditional organofunctionalized aerogels for specific applications.

Silica gels are synthesized by the hydrolysis and condensation ofsilicon containing precursors (in most cases an alkoxide). The sol canbe cast on fiber reinforcements to make flexible aerogel blankets, or asmonoliths in molds, or as particles or beads. Gelation can be tuned tooccur in seconds, minutes or hours, depending on the type and amount ofcatalyst used. The initial gels prepared are strengthened by an agingprocess at moderate temperatures during which hydrophobicity impartingagents can be added. Typical hydrophobicity imparting agents arehexamethyldisilazane (HMDZ) and hexamethylsiloxane (HMDS). Aerogels areobtained when the gels are dried in a manner that does not significantlyalter the structure of the wet gel. Supercritical processes (using CO₂)are used as they eliminate capillary forces that cause the gel structureto collapse. In the current process, the gels are made in alcoholsolvents and dried using supercritical carbon dioxide.

The method for making hydrophobic aerogels of the current inventioninvolves first making a gel. Subsequently, this preformed gel is soakedin a bath containing a mixture of solvent and the desired hydrophobizingagent in a process often referred to as surface derivatization. Forexample, U.S. Pat. No. 5,830,387 (Yokogawa et al.) describes a processwhereby a gel having the skeleton structure of (SiO₂)_(n) was obtainedby hydrolyzing and condensing an alkoxysilane. This gel was subsequentlyhydrophobized by soaking it in a solution of a hydrophobizing agentdissolved in solvent. Similarly, U.S. Pat. No. 6,197,270 (Sonada et al.)describes a process of preparing a gel having the skeleton structure of(SiO₂)_(m) from a water glass solution, and subsequently reacting thegel with a hydrophobizing agent in a dispersion medium (e.g., a solventor a supercritical fluid).

U.S. Patent Application 2011/0240907 describes methods where the metaloxide precursor comprises an organosilane, e.g., a tetraalkoxysilanesuch as tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) or apre-polymerized silicon alkoxide such as ethyl polysilicate. In someembodiments, the organosilane comprises an alkyl-substitutedalkoxysilane, such as methyltrimethoxysilane (MTMOS) or HMDZ. Generally,during the gel formation process, the hydrophobic surface modifyingagent combines with the skeletal structure formed by the metal oxideprecursor to provide a hydrophobic surface. In some embodiments, thehydrophobic surface modifying agent is covalently bonded, ionicallybonded, or physically adsorbed to the metal oxide skeleton. Generally,the hydrophobic surface modifying agent comprises two functionalelements. The first element reacts with (e.g., covalently or ionically)or absorbs on to the metal oxide skeleton. The second element is ahydrophobic surface modifying agents such as organosilane,organosiloxane, organotin, and organo-phosphorus compounds.

Silica aerogels, due to their large surface area and high porosity, makeeffective sorbents; however, the rigid pore structure is verysusceptible to collapsing if it is penetrated with a heavy liquid.Aerogels are imparted with chemical treatments to make them hydrophobic,thus avoiding pore collapse from liquid water.

Methyltriethoxysilane (MTES) aerogels modified with an amine precursorshows strong potential for high CO₂ adsorption capacity and longdurability, even in high steam content flue gas. MTES aerogel isproduced by mixing MTES in ethanol solution. The amount of water used tohydrolyze the silica precursor has been shown to have a huge impact onphysical properties (pore structure, surface area, density, shrinkage).The polycondensation reaction (gelation) is completed by adding ammoniaas base catalyst. The gelation time is long and ranges between 5 and 24hours depending on EtOH/MTES and ammonia/MTES ratios.

Unlike amine grafted zeolites or other supported amine sorbents actuallyunder investigation for carbon capture application, MTES aerogels areintrinsically hydrophobic (water contact angle >150°). This property isvital for performance durability and multiple-cycle use of a solidsorbent in CO₂ capture. Even small amounts of moisture can negativelyimpact the CO₂ capture performance over time. In general, postcombustion flue gas contains: 12% CO₂, 74% N₂, 12% H₂O, 4% O₂. Becauseof this water content, hydrophobic sorbent material is stronglyrecommended for CO₂ capture.

The advantage of amine functionalized hydrophobic silica aerogels incomparison with mere impregnation of an amine onto the aerogel matrix,is that loss of amine can be largely or completely prevented. In orderto ensure suitability of hydrophobic methyltriethoxysilane (MTES)aerogel for CO₂ capture application, the amine must be bonded into thesilica aerogel backbone (MTES leads to intrinsically hydrophobicaerogels without additional hydrophobe treatment). The route forproducing this type of hybrid silica aerogel is to co-gel MTES silicawith an amine functionalized silica precursor. MTES and three differentaminoalkoxysilane precursors (mono-, di-, and tri-amine) were used inthe preparation of amine functionalized aerogel (AFAs). The aminosilanessuch as 3-aminopropylmethyldiethoxysilane, 3-aminopropyl-triethoxysilane(APTES), 3-aminopropyl-trimethoxysilane (APTMS),N-(2-aminoethyl)-3-aminopropyltriethoxysilane (AE-APTES),N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AE-APTMS),p-aminophenyl-trimethoxysilane,N-3-([amino(polypropylenoxy])-amino-propyl-trimethoxy-silane(aminoether), (3-trimethoxylsilylpropyl)diethylenetriamine (TMS-DETA),and trimethoxy-silane modified polyethyleneimine are commerciallyavailable. The following amine-containing trialkoxysilane precursors mayalso be used: Aminopropyltriethoxysilane (mono-amine) (APTES),Aminopropyltrimethoxysilane (mono-amine) (APTMS),N-(2-aminoethyl)-3aminopropyltriethoxysilane (di-amine) (AE-APTES),(3-trimethoxysilylpropyl)diethylenetriamine (tri-amine) (TMS-DETA)

The chemical structures of the candidate precursor materials are shownin FIG. 1.

FIG. 2 shows the synthetic route of the sol-gel preparation of AmineFunctionalized Aerogels (AFAs).

-   -   The gel quality is determined by the hydrolysis degree of the        amine precursor, water content in the co-gel formulation,        ammonia during gelation, and reaction temperature. Forty weight        percent of amine sol can co-gel with MTES with target density        higher than 0.05 g/cc. The higher the target density, the higher        the amount of amine sol can be used in the formulation to result        in higher amine loadings in the sorbent.

Amine Functionalized Aerogels

In order to ensure suitability of hydrophobic methyltriethoxysilane(MTES) aerogel for CO₂ capture application, amine must be graft into thechemical backbone (MTES leads to intrinsically hydrophobic aerogelwithout additional hydrophobe treatment). The route for producing thistype of hybrid silica aerogel is to co-gel MTES silica with an aminefunctionalized silica source.

The samples listed in Table 1 were prepared by methods illustrated inthe examples below. To prepare the gels, first sols were prepared as inA and then cogelled as in B. To prepare the gel a MTES/TEOS sol (Sols1-4) was mixed with an aminosilane sol (Sols 5-10) to obtain gels whichwere then aged and supercritically dried. The MTES/TEOS sol number andthe aminosilane sol number for each gel are listed in Table 1. Thoseskilled in the art will be able to prepare the aerogels listed in Table1 based on the 11 examples described below.

A. Hydrolysis of MTES and Amine Precursors

Hydrolysis time was 18-24 hours in order to ensure full hydrolysis ofthe amine precursors. MTES and amine precursor were hydrolyzedseparately. MTES was hydrolyzed in acidic medium for periods ranging 16hours to 24 hrs. The aminosilanes were hydrolyzed without using an acidor base catalyst. Water to silica ratios were tuned in the range of1.5-12 with a fixed solid content of 20%.

B. Co-Gel of Amine and MTES Hydrolyzed Sols

Once hydrolyzed, the two sols are mixed in specific proportions toobtain the desired concentration of functional groups. Alcohol (ethanol)is added to the mixture to fix the target density of the aerogel. Insamples with Aspen IDs ending in A, ammonia was added to the final solfor polycondensation. No ammonia was added to samples with Aspen IDsending in NA. The sol turns to gel after a certain period of time(called gel time). This time depends on many parameters such asAmine/MTES content, H₂O/MTES ratio in the co-gel formulation, NH₄OHcontent and target density. Description and sorption characteristics ofsamples examples listed in Table 1 are grouped in Table 5, Table 6, andTable 7.

Synthesis of Amine functionalized aerogels (AFA) sorbents involvessynthesis of wet gel of amine functionalized MTES/TEOS followed by agingand liquid CO₂ extraction. Wet gel of amine functionalized MTES/TEOS canbe obtained by co-gel of amine functionalized silane to form a hybrid orby surface treatment of MTES/TEOS gels with amines in the aging stage byimpregnation. The amines included but not limited to mono-amine,diamine, triamine, pentaamine, and polyamines. Water and ammoniaconcentration can be varied in different formulations.

The volume of the sol after hydrolysis varies under different reactionconditions and need to be monitored and measured carefully in order tomaintain the targeted density of the gels. The reason is that targetdensity of the co-gel wet gel is defined by the solid content from bothMTES/TEOS and APTES sols divided by the volume of the gel. Condensationfactor is used to measure the concentrating degree of the sol bydefining as the ratio of the volume of the sol after reaction to thestarting volume.

Sol 1. Preparation of MTES/TEOS Sol with 4.1 Wt % of TEOS Using oPACatalystTo make 1200 ml of MTES/TEOS sol, first mix 0.71 g ortho-phosphoric acid(oPA) with 279.1 ml water in a 2-Liter round bottomed flask. To thismixture add 876.3 ml MTES and 46.4 ml TEOS while stirring. Then refluxthe mixture at 77-78° C. for 20 hours and cool it to room temperaturebefore use. Condensing factor is close to 1.

Sol 2. Preparation of MTES Sol Using oPA Catalyst

To make 1400 ml of MTES sol, first mix 0.82 g ortho-phosphoric acid(oPA) with 326.3 ml water in a 2-Liter round bottomed flask. To thismixture add 1073.1 ml MTES while stirring. Then reflux the mixture at76-77° C. for 15 hours and cool it to room temperature before useSol 3. Preparation of MTES/TEOS Sol with 2.0 Wt % of TEOS Using HOAcCatalystTo make 500 ml of MTES/TEOS sol, first mix 0.071 g Acetic acid (HOAc)with 116.3 ml water in a 1-Liter beaker. To this mixture add 374.0 mlMTES and 9.6 ml TEOS while stirring. Then heat the mixture to 60° C. for18 hours and cool it to room temperature before use. Final volume of thesol is 445 ml after cool to RT due to evaporation of solvent andcondensing factor is 0.89. Condensing factor of the sol was ˜1.Sol 4. Preparation of MTES/TEOS Sol with 4.0 Wt % of TEOS Using HOAcCatalystTo make 500 ml of MTES/TEOS sol, first mix 0.071 g Acetic acid (HOAc)with 116.1 ml water in a 1-Liter beaker. To this mixture add 364.8 mlMTES and 19.0 ml TEOS while stirring. Then heat the mixture to 60° C.for 18 hours and cool it to room temperature before use. Final volume ofthe sol is 367 ml after cool to RT due to evaporation of solvent andcondensing factor is 0.734. Condensing factor of the sol was ˜1.Sol 5. Preparation of APTES Sol with H₂O/Si=8To make 150 ml APTES sol, first mix 37.9 ml water and 54.5 ml ethanol.To this mixture add 57.6 ml APTES and stir for at least 24 hours at roomtemperature or heating for 6 hours at 60. ° C. Keep the container sealedduring the reaction. Condensing factor of the sol was ˜1 or 0.95 in twodifferent batches.Sol 6. Preparation of APTES Sol with H₂O/Si=2.2To make 50 ml APTES sol, first mix 3.3 ml water and 28.4 ml ethanol. Tothis mixture add 18.3 ml APTES and stir for at least 24 hours at roomtemperature or heating for 6 hours at 60° C. Keep the container sealedduring the reaction. Condensing factor of the sol was ˜1.Sol 7. Preparation of APTMS Sol with H₂O/Si=3To make 150 ml APTES sol, add 35.4 ml water to 114.6 ml APTMS in a 1L-beaker by adding drop wise and stir for 10 minutes at roomtemperature. This reaction is violent and should be controlled carefullyby slow addition of water and also cooling by ice bath if running hugevolumes. After the reaction, sol volume is reduced slightly and thenadded ethanol to the sol to reach 150 ml starting volume. In this case,condensing factor of the sol was ˜1.Sol 8. Preparation of AE-APTMS Sol with H₂O/Si=8

To make 230 ml APTES sol, first mix 38.3 ml water and 132.1 ml ethanol.To this mixture add 59.6 ml AE-APTMS and stir for 20 hours at roomtemperature. Keep the container sealed during the reaction. Condensingfactor of the sol was ˜1.

Sol 9. Preparation of AE-APTMS Sol with H₂O/Si=5.5To make 230 ml APTES sol, first mix 26.0 ml water and 145.2 ml ethanol.To this mixture add 58.8 ml AE-APTMS and stir for 20 hours at roomtemperature. Keep the container sealed during the reaction. Condensingfactor of the sol was ˜1.

Sol 10. Preparation of TMS-DETA Sol

Five (5) g of (3-trimethoxysilylpropyl) diethylenetriamine (TMS-DETA)were dissolved in 12 ml of EtOH. 3 ml of water were added to the mixtureand mixed for 16 hrs at room temperature.

The above sols were used to prepare the aerogels investigated as CO₂sorbents according to the following illustrative examples.

Example 1 Preparation of Sorbent CQ (WD-24C, Target Density: 0.065 g/Cc,40% APTES, Total H₂O/Si=4.4)

To make a gel with 40 ml volume, first dilute 6.1 ml MTES/TEOS solprepared in Sol 1 with 28.2 ml ethanol. To this mixture add 5.7 ml APTESsol prepared in Sol 5 while stirring. After stirring for 1 hour, pourthe sol into a mold for gelation. Gel time is within two days.

CO₂ sorption capacity for this sorbent at 40° C. and 0.15 PCO₂ is 6.74lb CO₂/100 lb sorbent with a fast sorption kinetics of 6.2 minutes to80% Capacity Equilibrium. Working Capacity of this sample is 4.35 lbCO₂/100 lb sorbent. Working Capacity is the capacity of the sorbent toadsorb the CO2 in the next capture cycle. This is typically thedifference between adsorption and desorption amount sin the immediatelypreceding cycle.

CO₂ sorption capacity for this sorbent at 55° C. is 7.40 lb CO₂/100 lbsorbent with a fast sorption kinetics of 4.2 minutes to 80% CapacityEquilibrium.

Example 2 Preparation of Sorbent CN (WD-25A, Target Density: 0.05 g/Cc,30% APTES, Total H₂O/Si=8)

To make a gel with 40 ml volume, first mix 5.5 ml MTES/TEOS sol preparedin Sol 1 with 31.2 ml ethanol while stirring. To this mixture add 3.3 mlAPTES sol prepared in Sol 5 and stir for 1 hour. Then pour the sol intoa mold for gelation. Gel time is within 2 days.

CO₂ sorption capacity for this sorbent at 55° C. is 7.26 lb CO₂/100 lbsorbent with a fast sorption kinetics of 5 minutes to 80% CapacityEquilibrium.

Example 3 Preparation of Sorbent CR (WD-26C, Target Density: 0.065 g/Cc,40% APTES, Total H₂O/Si=4.4)

To make a gel with 40 ml volume, first dilute 6.1 ml MTES sol preparedin Sol 2 with 28.2 ml ethanol. To this mixture add 5.7 ml APTES solprepared in Sol 5 while stirring. After stirring for 1 hour, pour thesol into a mold for gelation. Gel time is within four days.

CO₂ sorption capacity for this sorbent at 55° C. is 5.49 lb CO₂/100 lbsorbent with a fast sorption kinetics of 5 minutes to 80% CapacityEquilibrium.

Example 4 Preparation of Sorbent GE (WD-44-15C, Target Density: 0.262g/Cc, 40% APTES, Total H₂O/Si=1.6, 4% TEOS in MTES/TEOS Sol)

To make a gel with 75 ml volume, Mix 33.6 ml MTES/TEOS sol (withcondensing factor of 0.734) prepared in Sol 4 with 41.4 ml APTES sol(with condensing factor of 0.95) prepared in Sol 5. After stirring for10 minutes, pour the sol into a mold for gelation. Gel time is abouthalf an hour.

CO₂ sorption capacity for this sorbent at 40° C. and 0.15 PCO₂ is 7.37lb CO₂/100 lb sorbent with a fast sorption kinetics of 6.9 minutes to80% Capacity Equilibrium. Working Capacity of this sample is 5.60 lbCO₂/100 lb sorbent.

Example 5 Preparation of Sorbent HI (WD-48A, Target Density: 0.317 g/Cc,40% APTMS, Total H₂O/Si=1.86, 2% TEOS in MTES/TEOS Sol)

To make a gel with 150 ml volume, mix 105.6 ml MTES/TEOS sol (withcondensing factor of 0.952) prepared in Sol 3 and 33.9 ml APTMS sol(with condensing factor of 1) prepared in Sol 7. After stirring for 10seconds, pour the sol into a mold for gelation. Gel time is one minute.

CO₂ sorption capacity for this sorbent at 40° C. and 0.15 PCO₂ is 10.4lb CO₂/100 lb sorbent with a medium sorption kinetics of 39.1 minutes to80% Capacity Equilibrium. Working Capacity of this sample is 5.36 lbCO₂/100 lb sorbent.

Example 6 Preparation of Sorbent HH (WD-48B, Target Density: 0.331 g/Cc,50% APTMS, Total H₂O/Si=2, 2% TEOS in MTES/TEOS Sol)

To make a gel with 150 ml volume, mix 92.0 ml MTES/TEOS sol (withcondensing factor of 0.952) prepared in Sol 3 and 58.0 ml APTMS sol(with condensing factor of 1) prepared in Sol 7. After stirring for 10seconds, pour the sol into a mold for gelation. Gel time is one minute.

CO₂ sorption capacity for this sorbent at 40° C. and 0.15 PCO₂ is 9.6 lbCO₂/100 lb sorbent with a medium sorption kinetics of 30 minutes to 80%Capacity Equilibrium.

Example 7 Preparation of sorbent HG (WD-48C, target density: 0.347 g/cc,60% APTMS, total H₂O/Si=2, 2% TEOS in MTES/TEOS sol)

To make a gel with 150 ml volume, mix 77.1 ml MTES/TEOS sol (withcondensing factor of 0.952) prepared in Sol 3 and 72.9 ml APTMS sol(with condensing factor of 1) prepared in Sol 7. After stirring for 10seconds, pour the sol into a mold for gelation. Gel time is two minutes.

CO₂ sorption capacity for this sorbent at 40° C. and 0.15 PCO₂ is 8.4 lbCO₂/100 lb sorbent with a slow sorption kinetics of 46.9 minutes to 80%Capacity Equilibrium.

Example 8 Preparation of Sorbent WD-37A2 (Target Density: 0.065 g/Cc,30% AE-APTMS, Total H₂O/Si=8, No NH₄OH Catalyst in Gelation)

To make a gel with 50 ml volume, first mix 8.9 ml MTES/TEOS sol preparedin Sol 1 with 32.2 ml ethanol and 3.4 ml water while stirring. To thismixture add 5.5 ml AE-APTMS sol prepared in Sol 8 and stir for 1 hour.Then pour the sol into a mold for gelation. Gel time is within 5 days.

Example 9 Preparation of Sorbent WD-42C (Target Density: 0.087 g/Cc, 30%AE-APTMS, Total H₂O/Si=3.4, No NH₄OH Catalyst in Gelation)

To make a gel with 50 ml volume, first mix 11.9 ml MTES/TEOS solprepared in Sol 1 with 30.6 ml ethanol. To this mixture add 7.5 mlAE-APTMS sol prepared in Sol 9 and stir for 1 hour. Then pour the solinto a mold for gelation. Gel time is within 2.5 days.

Example 10 Preparation of Sorbent TA35W12NA, Target Density: 0.1 g/Cc,35% TMS-DETA, Total H₂O/Si=12, 4% TEOS in MTES/TEOS Sol

The MTES/TEOS (17.64 ml) sol prepared in example 4 was diluted with16.64 ml ethanol. To this mixture 5 ml of water were added. The solresulting was stirred for 1 hr. Later, TMS-DETA Sol 10 and dilutedMTES/TEOS sol were mixed together for 15 minutes before transfer intomold for gelation. Gelation occurred within 72 hours at roomtemperature. Gels were relatively weak.

TABLE 1 Preparation of amine functionalized aerogels. % MTES- Sample TD% Sol TEOS ID Aspen ID g/cc Amine Amine No. sol Sol No. CM A40W12NA0.065 APTES 40 5 60 1 CN A35W12NA 0.120 APTES 35 5 65 1 CQ A40W12NA0.100 APTES 40 5 60 1 CR A40W3A 0.120 APTES 40 6 60 2 CZ A30W12NA 0.100APTES 30 5 70 4 DF A40W12A 0.100 APTES 40 5 60 4 A25W12NA 0.087 APTES 255 75 4 A30W12NA 0.087 APTES 30 5 70 4 A40W6A 0.087 ATPES 40 6 60 4 CODA40W12NA 0.065 AE-APTMS 40 8 60 4 CP DA35W12NA 0.120 AE-APTMS 35 8 65 4CW DA40W12NA 0.10 AE-APTMS 40 8 60 4 CY DA40W3A 0.120 AE-APTMS 40 9 60 4DB DA30W12NA 0.100 AE-APTMS 30 8 70 4 CX DA40W3A 0.100 AE-APTMS 40 9 604 DA DA40W12A 0.100 AE-APTMS 40 8 60 4 DC DA40W6A 0.100 AE-APTMS 40 9 604 DD DA40W12A 0.065 AE-APTMS 40 8 60 4 DE DA35W12A 0.100 AE-APTMS 35 865 4 ED TA40W12NA 0.065 AE-APTMS 40 10 60 4 EE TA35W12NA 0.120 TMS-DETA35 10 65 4 EF TA35W12NA 0.100 TMS-DETA 35 10 65 4 EG TA40W3A 0.120TMS-DETA 40 10 60 4 EA TA30W12NA 0.100 TMS-DETA 30 10 70 4 EB TA30W12A0.100 TMS-DETA 30 10 70 4 EC TA25W12A 0.100 TMS-DETA 25 10 75 4 WD-34CTA40W6A 0.087 TMS-DETA 40 10 60 4 HI WD-48A 0.317 APTMS 40 7 60 3 HHWD-48B 0.331 APTMS 50 7 50 3 HG WD-48C 0.347 APTMS 60 7 40 3 HE B1untreated 0.250 PEI 50 — 40 4 GE WD-44-15C 0.262 APTES 40 5 60 4

Example 11 Preparation of Sorbent TA30W12A, Target Density: 0.1 g/Cc,30% TMS-DETA, Total H2O/Si=12, 4% TEOS in MTES/TEOS Sol

MTES/TEOS (20.58 ml) sol prepared in example 4 was diluted with 13.72 mlethanol. To this mixture 5 ml of ammonia solution (2 N) were added. Thesol resulting was stirred for 15 minutes. Noticeable increase inviscosity of the MTES/TOES sol was noticed. TMS-DETA sol (Sol 10) wasadded to MTES/TEOS sol and mixed together for 30 minutes before transferinto mold for gelation. Gelation occurred within 72 hrs at roomtemperature.

Physical and Structural Characterization

Some of the amine functionalized aerogels of the current invention(mono-, di-, and tri-amine/MTES aerogels) were analyzed by nitrogenadsorption-desorption technique. The composition of the aerogel samplestested is reported in Table 2.

TABLE 2 Composition of different Amine Functionalize Aerogels tested byliquid nitrogen-Sorption-desorption technique. Density Amine NH₄OHSample ID (g/cc) H₂O/Si⁽¹⁾ (wt %) (vol %) Mono-AFA A25W12NA 0.121 12 250 A30W12NA 0.131 12 30 0 A35W12NA 0.128 12 35 0 A40W12NA 0.124 12 40 0A40W3A 0.132 3 40 7 A40W6A 0.133 6 40 7 A40W12A 0.136 12 40 7 Di-AFADA40W6A 0.136 6 40 7 DA40W12A 0.134 12 40 7 Tri-AFA TA40W6A 0.142 5 40 7TA40W12A 0.133 12 40 7 ⁽¹⁾Total amount of water in the co-gel

FIG. 3 shows the N₂ adsorption isotherms of the mono-AFA samplesun-catalyzed. These aerogels exhibit type IV isotherms consistent with ahighly porous solid with an average pore size of around 9-10 nm. Theisotherm, as shown in FIG. 3, exhibits a significant hysteresis curve athigh partial pressures, a feature that is typically consistent withcapillary condensation within mesopores. Analysis of the adsorptionisotherm data will afford the surface area, average pore size andcumulative pore volume for these materials. As the amount of amineincreases, the surface area and pore volume of the aerogel decrease. Asillustrated in FIG. 4, the second set of mono-AFA samples (catalyzedwith ammonia) exhibit different isotherms with a volume of liquidnitrogen adsorbed at high partial pressure relatively lower than theprevious set of samples. The use of ammonia catalyst during gelation(and the amount of water in the gel) seems to have a big impact on thestructure of the aerogel. The high pH of the system make the structureof the aerogel relatively compact with small pore size distribution andlow pore volume.

FIG. 5 and FIG. 6 illustrate the pore size distribution of the two setof samples. The un-catalyzed samples have broader pore size distributioncentered on 20 nm. As the amine content increases, the pore volumeadsorbed decreases and average pore diameter remains unchanged. Thecatalyzed samples exhibit more narrowed pore size distribution (3-4 nm)and size of the pores get smaller as base is used as catalyst and wateramount increases in the gel. It should also be noted that N₂ adsorptionis most applicable for mesoporous solids and generally underestimatesthe specific surface area of aerogels as it inherently neglects anycontributions from micro- and macroporosity. The meso-structureproperties of the AFA samples are reported in Table 3.

TABLE 3 Composition of different Amine Functionalize Aerogels tested byliquid nitrogen-Sorption-desorption technique. Cum. Pore ~pore DensityS_(BET) volume diamter Sample ID (g/cc) (m²/g) (cc/g) (nm) Mono-AFAA25W12NA 0.121 420 0.92 10.6 A30W12NA 0.131 408 0.51 12.1 A35W12NA 0.128400 0.52 9.2 A40W12NA 0.124 264 0.63 9.3 A40W3A 0.132 405 0.41 4.1A40W6A 0.133 204 0.51 4.5 A40W12A 0.136 130 0.32 5.0 Di-AFA DA40W6A0.136 455 1.39 12.0 DA40W12A 0.134 323 1.25 16.0 Tri-AFA TA40W6A 0.142164 0.95 24.0 TA40W12A 0.133 25 0.23 39.2

Surface Nitrogen by X-Ray Photonelectron Spectroscopy (XPS) Measurements

Two AFA samples (CN and CQ) and one blank aerogel sample (pure MTESaerogel) were analyzed by X-ray Photonelectron Spectroscopy (XPS). Thecompositions of the three samples are given in Table 4.

TABLE 4 Composition of the aerogel samples subjected to XPS analysis. IDsample ID sample (by Amine Water/Si Density (by ADA)⁽¹⁾ Aspen) loading(wt. %)⁽²⁾ ratio (g/cc) CN A35W12NA 35 12 0.142 CQ A40W12NA 40 12 0.132— MTES 0 8 0.112 ⁽¹⁾CN and CQ AFA samples were tested by ADA for CO2capacity and energy regeneration. ⁽²⁾Amine loading corresponds to theamount by weight of APTES commixed with MTES.

The analysis was performed at Answer Analytical Inc. and carried outwith a PHI 5600ci instrument using monochromatic Al K_(α), X-rays. Thespectra of the three samples are shown in FIG. 7. Presence of amine onthe surface of the two AFA samples confirms the functionalization ofbackbone of MTES aerogel. The % nitrogen (N1s Peak) and other elements(Oxygen, O (1s), Silicon Si (2s, 2p), Carbon (1s)) were calculated bymeasuring peak areas in the high-resolution spectra and then convertingto atomic concentrations using instrument manufacturer providedsensitivity factors. The concentration of nitrogen in CQ sample isaround 9.7%, higher than CN sample by 44% (5.4% N in CN sample).

Testing the Sorbent

All testing was carried out using a specialized fixed bed reactor. Thisunit was designed to be used in the laboratory on simulated flue gas aswell as in the field on actual flue gas with minimal modifications. AProgrammable Logic Controller was employed to completely automate theprocess. With an automated system, a series of sorption/regenerationcycles can be completed with little to no supervision. The flow rate ofeither simulated or actual flue gas was approximately 300 sccm, and theamount of sorbent in the reactor was usually in the range of 0.5 to 2.5g, depending on each material's particle size. The sorbent and flue gasare contacted in a fixed bed through a sequence of temperaturecontrolled lines and electrically controlled valves. FIG. 8 shows aschematic of the sorbent screening testing unit when setup forlaboratory testing (only minor modifications are necessary for fieldtesting). The CO₂ analyzer was a continuous NDIR sensor with a 90%response time of 10 seconds. This response time should be taken intoconsideration when examining results. It is probable that the responsetime of the instrument affects results for materials tested in 0.5 gquantities more extensively than those tested in 2.5 g quantities.

After the sorbent was placed into the fixed bed it was heated to aninitial flushing temperature. The initial flushing temperature based onthe lowest regeneration temperature. A thermocouple on the outside ofthe glass fixed bed was used to determine when the bed had reaches thedesired temperature. When the bed temperature matches the desiredsorption temperature, the sorbent was flushed with dry N₂ for 10 minutesor until no CO₂ was measured in the purge gas stream, whichever waslonger. Then the simulated flue gas, an admixture of compressed gases,was sent through the bypass line circumventing the sorbent. Thecomposition of the laboratory sample gas, by volume, was approximately12% CO₂, 4% O₂, with a balance of N₂. The relative humidity (RH) wascontrolled using a heated bubbler. The most common RH set points are 0%or 50% (0 or ˜7% by volume, respectively). Note that the 7% moisture byvolume corresponds to a dew point (i.e. bubbler temperature setting) of40° C. When the CO₂ reading stabilized at the known CO₂ concentration(i.e., baseline reading), the gas flow was directed through the sorbent.After the CO₂ levels returned to their original levels (i.e., thesorbent was saturated with CO₂), the gas was sent through the bypass,which was the end of the sorption step.

A temperature swing with a N₂ purge gas was used to regenerate thesorbents and desorb the CO₂. The regeneration purge gas flow rate wasthe same as that of the flue gas, approximately 300 sccm. To begin theregeneration step, the system stopped flue gas flow and began heated N2flow. While the heated purge gas was flowing through the sorbent theheat tape on the outside of the fixed bed was used to ensure that thesorbent was fully heated to the selected regeneration temperature.Upstream of the reactor the N₂ purge gas was directed through a bubblerseparate from the one used for sorption. This bubbler was primarily usedat room temperature to add less than 2% by volume moisture to theregeneration gas.

Sorbent Testing Conditions

Three batches of different AFA samples were sent to ADA for CO₂ sorptioncapacity screening. The laboratory testing conditions of the threebatches are provided in the Table 5, Table 6, and Table 7.

TABLE 5 Sorbent screening conditions (mono-AFA) Sorbent (ADA# ID) ED EEEF EG EA EB EC Sorbent (Aspen #ID) TA40W12NA TA35W12NA TA40W12NA TA40W3ATA30W12NA TA30W12A TA25W12A Density 0.082 0.142 0.132 0.142 0.131 0.1350.133 (g/cc) Test type Parametric Parametric Parametric ParametricParametric Parametric Parametric Cycle completed 38 34 37 26 30 26 29Sample size (g) 1.08 1.01 0.49 1.03 1.01 1.03 0.99 Sorption conditionsCO₂ Conc. (%) 12 12 12 12 12 12 12 O₂ Conc. (%) 4 4 4 4 4 4 4 Moisturelevel (%) 40 40 40 40 40 40 40 Temperature (C.) 55 55 55 55 55 55 55Time (sec.) 250 250 250 250 250 250 250 Regeneration conditionsTemperature (C.) 80 (1-9)  80 (1-9)  80 (1-9)  80 (1-9)  80 (1-9)  80(1-9)  80 (1-9)  (cycles) 100 (10-19) 100 (10-19) 100 (10-19) 100(10-19) 100 (10-19) 100 (10-19) 100 (10-19) 120 (20-39) 120 (20-35) 120(20-38) 120 (20-27) 120 (20-31) 120 (20-27) 120 (20-30) Time (sec.) 750800 800 800 900 900 950

TABLE 6 Sorbent screening conditions (di-AFA) Sorbent (ADA# ID) CO CP CWCY DB Sorbent (Aspen #ID) DA40W12NA DA35W12NA DA40W12NA DA40W3ADA30W12NA Density 0.082 0.142 0.134 0.142 0.135 (g/cc) Test typeParametric Parametric Parametric Parametric Parametric Cycle completed34 110 39 33 42 Sample size (g) 0.5 0.54 0.53 1.07 1.09 Sorptionconditions CO₂ Conc. (%) 12 12 12 12 12 O₂ Conc. (%) 4 4 4 4 4 Moisturelevel (%) 50 50 50 50 50 Temperature (C.) 55 55 55 55 55 Time (sec.) 300300 300 300 250 Regeneration conditions Temperature (C.) 80 (1-9)  80(1-19)  80 (1-9)  80 (1-9)  80 (1-9)  (cycles) 100 (10-19) 100 (20-40) 100 (10-19) 100 (10-19) 100 (10-19) 120 (20-35) 120 (41-125) 120 (20-41)120 (20-34) 120 (20-43) Time (sec.) 850 650 700 800 1800 Sorbent (ADA#ID) CX DA DC DD DE Sorbent (Aspen #ID) DA40W3A DA40W12A DA40W6A DA40W12ADA35W12A Density 0.133 0.136 0.135 0.096 0.132 (g/cc) Test typeParametric Parametric Parametric Parametric Parametric Cycle completed90 30 32 32 39 Sample size (g) 1.16 1.02 1.02 1 1 Sorption conditionsCO₂ Conc. (%) 12 12 12 12 12 O₂ Conc. (%) 4 4 4 4 4 Moisture level (%)50 50 50 50 50 Temperature (C.) 55 55 55 55 55 Time (sec.) 300 400 250300 250 Regeneration conditions Temperature (C.) 80 (1-17) 80 (1-9)  80(1-9)  80 (1-9)  80 (1-9)  (cycles) 100 (18-21) 100 (10-19) 100 (10-19)100 (10-19) 100 (10-19) 120 (22-91) 120 (20-31) 120 (20-32) 120 (20-33)120 (20-40) Time (sec.) 700 950 700 800 800

TABLE 7 Sorbent screening conditions (tri-AFA) Sorbent (ADA# ID) CM CNCQ CR CZ DF Sorbent (Aspen #ID) A40W12NA A35W12NA A40W12NA A40W3AA30W12NA A40W12A Density 0.082 0.142 0.132 0.142 0.135 0.135 (g/cc) Testtype Parametric Parametric Parametric Parametric Parametric ParametricCycle completed 41 41 36 34 57 23 Sample size (g) 0.51 0.48 0.51 0.531.1 1 Sorption conditions CO₂ Conc. (%) 12 12 12 12 12 12 O₂ Conc. (%) 44 4 4 4 4 Moisture level (%) 50 50 50 50 50 50 Temperature (C.) 55 55 5555 55 55 Time (sec.) 400 300 250 300 400 300 Regeneration conditionsTemperature (C.) (cycles) 80 (1-9)  80 (1-9)  80 (1-9)  80 (1-9)  80(1-9)  80 (1-9)  100 (10-19) 100 (10-19) 100 (10-19) 100 (10-19) 100(10-19) 100 (10-19) 120 (20-42) 120 (20-42) 120 (20-37) 120 (20-35) 120(20-58) 120 (20-24) Time (sec.) 550 600 600 800 1800 1500

Sorbent Testing Results

Based on the laboratory test results, the CO₂ capacity and the energyfor sorption were estimated. Regeneration energy was calculated byestimating specific heat of the sorbents around 1.3 kJ/kg-K, an enthalpyof reaction of −60 kJ/mol CO₂, and a regeneration temperature of 100° C.Overall, the AFA sorbents showed a high CO₂ capacity and excellent CO₂sorption/desorption cycling stability, with sample CQ showing thehighest CO₂ capacity and lowest regeneration energy. The sections belowdescribe these results in more detail.

CO₂ Capacity and Energy of Regeneration Measurement Mono-AmineFunctionalized Aerogels

Based on the laboratory test results, the theoretical regenerationenergy for sorbents tested was estimated to be less than 1500 kJ/kg CO₂.The calculation of the regeneration energy was made by estimatingspecific heat of the sorbents around 1.3 kJ/kg, an enthalpy of reactionof CO2-sorbent reaction of −60 kJ/mol CO₂, and a regenerationtemperature of 100° C. Based on these results, CQ (A40W12NA) seems topossess the highest CO₂ capacity and lowest regeneration energy. CM, CN,and CQ materials exhibit a high CO₂ capacity cycling stability, and theyare considered for further testing. These samples contain 35-40% wtamine and differ by their density. Catalyzed samples CR and DF (A40W3Aand A40W12A) demonstrate low and relatively unstable CO₂ capacity overcycling. The hypothesis given previously saying that un-catalyzedsamples with their open and broader pore size distribution facilitatethe access of CO₂ to the amine sites (which results in high CO₂capacity), was proven to be correct if we compare the results for thesamples DF and DR (un-catalyzed) with the others. Overall, themono-amine (APTES) functionalized aerogels (un-catalyzed) show a highCO₂ capacity and excellent CO₂ sorption/desorption cycling stability.

Di-Amine Functionalized Aerogels

Di-amine (AE-APTES) functionalized aerogels were tested, at the sameconditions as the mono-amine samples. Sample CW (DA40W12NA),un-catalyzed and contains 40 wt % amine, seems to be the best sorbentamong the others in terms of capacity and stability of CO₂ capture(˜6.59%). From this batch of samples, we can make the following remarks:

-   -   Un-catalyzed samples (CO, CP, CW, and DB): The CO₂ capacity        strongly depends on density and amine content. Un-catalyzed        di-AFA material with 40% amine loading and a density of 0.13        g/cc (CW: DA40W12NA) is considered a potential candidate for CO₂        capture with CO₂ capacity around 6.59 wt %. Note that the water        content for this specific sample is 12 (H₂O/SiO₂ ratio).    -   Effect of water content (CX, DA, DC): with same amine loading        (40%) and same final density, these di-AFA samples showed        different CO₂ capacity. Low water content (H₂O/SiO₂=3 for sample        CX, DA40W3A) leads to poor CO₂ capacity. As water content        increases, CO₂ capacity also increases since higher water        contents lead to complete hydrolysis and co-polycondensation of        MTES and di-amine precursor (AE-APTES). H₂O/SiO₂ ratio of 12        showed to be the optimum for max. CO₂ capacity outcome.    -   Effect of density: Low density di-AFA materials (CO, DD)        demonstrated low CO₂ capacity compared to the ˜0.13 g/cc        samples. This can be explained by low amine content overall in        the material (although they contain 40 wt % amine).

Tri-Amine Functionalized Aerogels

The batch of tri-amine (TMS-DETA) Functionalized Aerogels was testedrelatively at the same conditions as the two previous batches (exceptthe humidity which was fixed at 40% instead 50%). The performance of thesamples was lower than expected. They exhibit very low CO₂ capacity (notexceeding 2 wt %). The effect of amine content, density and watercontent in the sol-gel preparation don't seem to have big impact on theCO₂ capture performance. This specific tri-amine precursor will not beconsidered for further development and optimization since the aerogelstructure does not seem to be retained when using this amine.

FIG. 9 summarizes the performance of all AFA sorbents. It's clear thatthe mono-AFA materials demonstrated the optimum performance in terms ofhigh CO₂ capacity and lower energy of regeneration. The energy ofregeneration is inversely proportional to the working CO₂ capacity ofthe sorbent. The higher the working capacity, the lower the energy forregeneration, since there is less non-active mass going through thedesorption cycles. Some of diamine and triamine functionalized aerogelsare also good CO₂ capacity sorption while some others did not.

Thermal Gravimetric Analysis (TGA) Under CO₂

The sample CQ was tested by TGA under CO₂ gas to evaluate working CO₂capacity of the material. The sample was heated at differenttemperatures and CO₂ partial pressure. The working CO₂ capacity, which avery important property to assess the performance of sorbent, is thedifference between the CO₂ capacity at temp. of sorption and temp. ofregeneration. Typically, a sorbent with working capacity of 5% orgreater is considered as promising sorbent for further tests. SampleCQ's performance, as shown in FIG. 10, looks very promising at partialpressure of CO₂ of 0.8. The working capacity approached 3%.

New materials have been prepared and tested and sample GE has beentested for >1000 cycles and it CO₂ sorption capacity is higher than CQas shown in FIG. 11. The working capacity of sample GE is also superiorand is shown in FIG. 12 and is 6.3% which is very good compared to otheramine supported sorbents.

Long-Term CO₂ Capture Sorption/Desorption of CQ Material.

The AFA sorbents are one of the most robust families of sorbentsevaluated and have been cycled more than 275 times in thesorption/desorption test facility with little to no loss in CO₂capacity. In Figure, the sorption profile from cycles 1 through 275 forsample CQ essentially is unchanged. Plus, this AFA material has shownhigh thermal stability and high thermal oxidation resistance. After testwas completed, it was noticed that the aerogel sample (AFA, CQ material)was not discolored (typical of stable sorbents). Unstable sorbents,which most of them are, turn yellow due to oxidation after long termtesting. This characteristic of the aerogel sorbents adds to the valueof the AFA material and makes them very competitive sorbents for CO₂capture.

The New AFA Sorbents are being Evaluated and have been Cycled More than1000 Times in the Sorption/Desorption Test Facility with Little to NoLoss in CO₂ Capacity.

In Figure, the sorption profile from cycles 1 through 1100 for sample GEessentially is unchanged. Plus, this AFA material has shown high thermalstability and high thermal oxidation resistance.

TABLE 8 Capacity and regeneration energy of all AFA samples. Amine TypeTRE (Regeneration energy) Sorbent NH₃ Weight CO₂ Capacity [MONO, DI,[kJ/kgk] [ID] catalyst [grams] [wt %] TRI] Low: Cp = 0.7 High: Cp = 1.3CQ No 0.51 7.4 MONO 1827 2192 CN No 0.48 7.3 MONO 1836 2207 CO No 0.506.6 DI 1880 2289 CW No 0.53 6.6 DI 1880 2289 CR Yes 0.53 5.5 MONO 19752467 CZ Yes 1.10 5.4 MONO 1989 2493 CP No 0.54 4.9 DI 2042 2591 CY Yes1.07 4.9 DI 2049 2603 DC Yes 1.02 4.4 DI 2124 2743 CM No 0.41 4.2 MONO2145 2781 DA Yes 1.02 4.0 DI 2187 2861 DF Yes 1.00 2.7 MONO 2568 3568 DDYes 1.00 2.3 DI 2771 3945 DE Yes 1.00 1.7 DI 3288 4905 EF No 0.49 1.6TRI 3383 5081 EA No 1.01 1.3 TRI 3825 5902 EG Yes 1.03 1.3 TRI 3882 6008EE No 1.01 0.9 TRI 4789 7692 CX Yes 1.16 0.8 DI 5546 9099 DB Yes 1.090.7 DI 5717 9415 EC Yes 0.99 0.7 TRI 5902 9759 EB Yes 1.03 0.6 TRI 648210837 ED No 1.08 0.6 TRI 7129 12038

1. A process of preparing a carbon dioxide capture sorbent comprisingthe steps of hydrolyzing at least an alkylalkoxysilane, reacting thehydrolyzed alkylalkoxysilane with at least a hydrolyzed aminosilane toform a gel and drying the resulting gel to obtain an aerogel, whereinthe aerogel comprises at least an open pore accessible to carbondioxide.
 2. The process of claim 1 wherein at least an amino group isaccessible to carbon dioxide.
 3. The process of claim 1 wherein theaerogel is hydrophobic.
 4. The process of claim 1 further comprising thestep of reacting a hydrolyzed tetra-alkoxysilane with at least one ofthe hydrolyzed silanes.
 5. The process of claim 1 further comprising thestep of reacting a dialkyldialkoxysilane with at least one of thehydrolysed silanes.
 6. The process of claim 1 wherein the alkyl groupcontains between 1 and 6 carbon atoms.
 7. The process of claim 1 whereinany of the silanes have additional functional groups.
 8. The process ofclaim 1 wherein the alkylalkoxysilane contains a mono, di or tri alkylgroups and selected from the group consisting of methyltrimethoxysilane,methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane,propyltrimethoxysilane, propyltriethoxysilane, dimethyldimethoxysilane,dimethyldiethoxysilane, diethyldiethoxysilane, diethyldimethoxysilane,trimethylmethoxysilane, trimethylethoxysilane, triethylmethoxysilane,triethylethoxysilane, tripropylmethoxysilane, tripropylethoxysilane,(3,3,3-Trifluoropropyl)trimethoxysilane,(3,3,3-Trifluoropropyl)triethoxysilane and a combination thereof.
 9. Theprocess of claim 1 wherein the aminosilane comprises mono, di, tri orpoly amine groups.
 10. The process of claim 1 wherein the aminosilane isselected from the group consisting of 3-aminopropylmethyldiethoxysilane,3-aminopropyl-triethoxysilane (APTES), 3-aminopropyl-trimethoxysilane(APTMS), N-(2-aminoethyl)-3-aminopropyltriethoxysilane (AE-APTES),N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AE-APTMS),p-aminophenyltrimethoxysilane,N-3-([amino(poly-propyleneoxy])-amino-propyl-trimethoxy-silane(aminoether), (3-trimethoxylsilylpropyl)-diethylenetriamine (TMS-DETA),trimethoxy-silane modified polyethyleneimine and a combination thereof.11. The process of claim 1 wherein at least an aminogroup is located onor adjacent to the pore surfaces.
 12. The process of claims 1 furthercomprising the steps of contacting the sorbent with a gaseous streamcomprising at least some carbon dioxide and capturing at least some ofthe carbon dioxide in the sorbent.
 13. The process of claim 12 furthercomprising a step of regenerating the sorbent.
 14. The process of any ofclaim 13 wherein the sorbent is stable for at least 250 or at least 500or at least 1000 or at least 2000 capture-regeneration cycles.
 15. Theprocess of claim 1 wherein the aminosilanes from 5% to 70% by weight ofthe total reactants.
 16. The process of claim 4 wherein the percentageof tetra-alkoxysilane in the total silanes is between 5 and 90%.
 17. Theprocess of claim 1 wherein the density of the aerogel is between 0.01and 0.6 g/cc and preferably between 0.03 and 0.34 g/cc.
 18. The processof claim 12 wherein the carbon dioxide capture rate is between 0.08 gramand 0.5 gram of carbon dioxide per gram of the aerogel in the sorbent.19. The process of claim 12 wherein any degradation in capture rate isno more than 80% after exposing to temperatures up to 130° C. andcapture-regeneration cycles up to
 500. 20. A process of preparing acarbon dioxide capture sorbent comprising the steps of hydrolyzing andcondensing at least an alkylalkoxysilane, forming a gel, reacting thegel with at least an amine or amine containing compound, drying theresulting gel to obtain an aerogel, wherein the aerogel comprises atleast an open pore accessible to carbon dioxide.
 21. A process ofpreparing a carbon dioxide sorbent comprising surface-treating a wet gelor an aerogel prepared from an alkylalkoxysilane with an amine:
 22. Theprocess of claim 20 where the amine is selected fromtetraethylenepentamine (TEPA), polyethyleneimine (PEI) or combinationsthereof.